The disclosure concerns a device and a method to improve the print quality of an ink printer, in particular what is known as a drop-on-demand ink printer, in which individual drops are generated and expelled from a nozzle only as needed.
In such drop-on-demand ink printers, the danger exists that ink starts to dry in the nozzles after longer periods of non-use of the nozzle, and this can lead to the clogging of the nozzle. Since a portion of the fluid evaporates more and more over time, this leads to an increase of the viscosity, whereby an ink droplet to be expelled takes somewhat longer until it is expelled or the ink flow velocity is reduced. Such a droplet therefore strikes the recording medium later. If a nozzle is clogged, an image point (pixel) will not be printed at all. Print errors can arise to a significant degree as a result of this.
Numerous methods as to how the clogging of nozzle channels of ink printers can be prevented are known from the prior art. From EP 1 038 677 A1 a method is known in which every nozzle is observed as to how long it has been since an ink drop has been expelled. If this length (what is known as a dead time) is greater than a predetermined limit value, the nozzle or the ink print head is moved into a park position in which the nozzle is then flushed with a larger ink drop.
A method to avoid the drying out of nozzles is known from the disclosure document DE 10 2007 035 805 A1. The ink print head is thereby not moved into a cleaning position; rather, the cleaning of the nozzles is conducted during the print operation. For this ink droplets are emitted from nozzles according to a predetermined algorithm. These ink droplets overlap on the recording medium with an image point that has already been printed beforehand or an image point that is still to be printed at the same point. All seldom used nozzles are thus always flushed with ink again and cleaned without the print image being conspicuously visibly affected.
It is also known that some image points on one side of the nozzles are printed at arbitrary, random points on the recording medium. A certain “noise” of image points is thus created in the background that should hardly stand out, however. Nevertheless, a degradation of the image quality exists, in particular given high graphical requirements.
A method to flush the nozzles is likewise known from the patent document U.S. Pat. No. 6,561,622 B1. In this an ink print head with the nozzles is moved into a park position and there the nozzles are flushed through with different ink volumes.
All of these known methods deal with the cleaning of the nozzles. There it is thus prevented that ink channels dry out completely. After a short non-use period of a nozzle, the viscosity of the ink can already critically increase so much that errors can be established in the print image as a result of the altered viscosity of the ink.